For the production of powder of a vinyl polymer to be used as a modifier such as impact resistance improver or flame retardant in a blended form with a synthetic resin or an elastomer, a method using a dispersion polymerization in an aqueous medium such as emulsion polymerization is widely used. When emulsion polymerization is used as a method for producing a vinyl polymer, a process for recovering the vinyl polymer as a solid such as powder from an obtained latex of the vinyl polymer is necessary.
As the above-mentioned process, a process of mixing an acid or a salt with the latex to obtain a coagulated slurry followed by heat treatment, dehydration and drying (hereinafter, referred to as “coagulation method”) has been widely known. Although this method can be applied to a latex which can be coagulated by an acid or a salt, this method cannot be applied to a latex which is difficult to coagulate such as one in which a nonion surfactant is used.
In addition, in the coagulation method, plural steps such as coagulation, heat treatment, dehydration, and drying are necessary, and the equipment becomes complicated. Further, there is a case where a system for controlling a coordination among the aforementioned plural steps is necessary. Consequently, improvement of a process for recovering powder of a vinyl polymer is desired from the viewpoint of cost of equipment or operation.
In addition, in the coagulation method, a large amount of waste water containing water soluble components derived from components mainly used in the polymerization step and the coagulation step generates in the dehydration step, and an apparatus for the treatment of the waste water is necessary, and thus the total equipment becomes large.
To solve these problems, a method of obtaining powder by spraying a latex of the vinyl polymer from a nozzle while drying it by hot air (hereinafter, referred to as “spray drying method”) is widely known. In the spray drying method, coagulation is not needed, so that even a latex difficult to coagulate can be recovered as powder, and moreover, the equipment is simplified because spraying and drying can be carried out in one step. Further, waste water does not generate, so that a device for waste water treatment is not needed, and thus there is an advantage such that the cost of equipment is low.
On the other hand, an anion emulsifier such as sodium alkylbenzene sulfonate as an emulsifier and a sulfonic acid or sulfuric acid as an acid catalyst are generally used in the production of the latex of the silicone polymer.
Therefore, in the powder which is recovered by use of the spray drying method from the latex of the vinyl polymer containing the silicone polymer obtained by polymerization of a vinyl monomer in the presence of the latex of the silicone polymer, a large amount of sulfates derived from the emulsifier and the acid catalyst is remaining.
In a shaped article obtained by blending of such powder of the vinyl polymer containing the silicone polymer with a polycarbonate resin (hereinafter, referred to as “PC resin”), hydrolysis of PC resin is accelerated by the sulfates remaining in the powder and thus the physical properties of the shaped article are considerably deteriorated.
To solve this problem, for example, a method of blending powder of a vinyl polymer obtained by spray drying of a latex of a vinyl polymer containing a silicone polymer having a specified composition with a thermoplastic resin such as PC resin to prevent lowering of breaking strength of the shaped article after wet heat treatment and to prevent lowering of hydrolysis resistance of PC resin is shown (Patent Document 1).
However, the condition of the wet heat treatment shown in Patent Document 1 is mild and thus it is not sufficient for the evaluation of hydrolysis resistance. The powder of a vinyl polymer shown in Patent Document 1 contains a large amount of sulfates and does not have enough effect for prevention of lowering of hydrolysis resistance of PC resin.
In addition, a method of blending powder of a vinyl polymer obtained by spray drying of a latex obtained by emulsion polymerization of a mixture of a silicone macromonomer and a vinyl monomer with a thermoplastic resin such as PC resin is shown in Patent Document 2.
Although a remaining amount of sulfates in the powder of a vinyl polymer shown in Patent Document 2 is small, it is presumed that the powder does not have sufficient improvement effect on impact resistance of the shaped article to be obtained and rather lowers the impact resistance because the molecular weight of the silicone macromonomer is low.